Magnetic alloy for data storage devices



United States Patent U.S. Cl. 75-170 2 Claims ABSTRACT OF THE DISCLOSURE This is an invention of a magnetic alloy having a substantially rectangular hysteresis loop, a coercive force of not more than 3 oersteds and substantially magnetically isotropic in which said alloy contains 65-83% nickel, 15-33% cobalt and 2% phosphorus.

This invention relates to improved processes of preparing coatings of magnetisable material, particularly by controlled autocatalytic chemical reduction precipitation of metallic ions on a catalytically active substrate i.e. by so-called electroless plating, and also to the improved coatings produced thereby.

It is an object of the invention to produce magnetisable electroless nickel-cobalt coatings for use in the fabrication of magnetic data storage devices.

According to one feature of the invention there is provided an autocatalytic chemical reduction process for continuously plating a nickel-cobalt magnetisable coating on metallic or n0n-rnetallic objects, which includes providing an aqueous solution including NiCl 6H O in the concentration of 11.5-22.5 grams/litre of the solution, CoCl 6H O in the concentration of 12.5-19 grams/litre of the solution, an alkaline hypophosphite, sodium citrate and ammonium chloride, immersing an object to be plated into the solution for a period of time and then removing the plated object from the solution, and maintaining the solution throughout the period of immersion at a temperature in the range of about 85 C. to 95 C. and at a pH value in the range of 5.8 to 6.4, when measured at 20 C., by the addition of ammonium hydroxide.

According to another feature of the invention there is provided an aqueous bath solution for the deposition of a nickel-cobalt magnetisable coating by autocatalytic chemical reduction precipitation on to a catalytically active substrate, the constituents for making up the bath solution including NiCl 6H O in the concentration of 11.5-22.5 grams/litre of the solution and CoCl 6H 0 in the concentration of 12.5-19 grams/ litre of the solution, and the bath solution having a pH in the range of 5.8-6.4 when measured at 20 C.

According to a further feature of the invention there is provided a magnetic alloy containing 65-83% nickel, 15-33% cobalt and 2% phosphorus, and which has a substantially rectangular hysteresis loop, a coercive force of not more than 3 oersteds, and is substantially magnetically isotropic.

A preferred example of a plating process according to the invention will now be described, using a plating solution of the composition given below.

Grns. Nickel chloride NiCl 6H 0 13.5 Cobalt chloride CoCl 6H O 17.5 Sodium hypophosphite NaH PO 14.9 Sodium citrate NH3C6H507SHZO Ammonium chloride NH Cl 40 Distilled water, to one litre of solution.

A copper sheet substrate is immersed in the above solution for 15 minutes while the temperature is maintained at 90 C. and at a pH of 6.0 when measured at 20 C. To start the plating process the copper surface is sensitized, i.e. catalytically activated, by touching it with an aluminum wire. This process produces a plated coating approximately 13,000 A. thick and having a composition of 68% nickel, 30% cobalt and 2% phosphorus. The coating is found to have magnetically isotropic square hysteresis loop properties, with a coefiicient of rectangularity of approximately 0.95 and a coercive force of 1.0 oersteds.

We have found that the coercive force of the coating varies with the temperature of the plating solution. For example, with the plating process described above the coercive force of the coating is approximately 2.5 oersteds for a solution temperature of C. or 95 C. and d creases for temperatures between these two values to a minimum of 1.0 oersteds at C.

We have also found that if the above plating solution is acidic with pH of between 5.8 and 6.4 the coating has isotropic properties with a coercive force of less than 2.5 oersteds. With a pH above 6.4 the coercive force increases rapidly, and with a pH of below 5.8 the coating is non-magnetic. With a pH of between 5.9 and 6.1 the coating has a coercive force of approximately 1.0 oersteds. The advantage of working with an acidic solution is that ammonia (in the form of ammonium hydroxide) need only be added to the solution while plating is actually taking place in order to keep the pH constant, as opposed to an alkali plating solution from which ammonia is lost continuously when heated.

The ratio of nickel to cobalt in the plating solution, and hence in the resultant coating, can be varied while still resulting in a coating with a high coetficient of rectangularity, i.e. above 0.95, and with a low value of coercive force, i.e. not more than 3 oersteds. Coatings with these properties can be attained with a solution having nickel concentration of from 11.5-22.5 grams/ litre of NiCl 6H O together with a cobalt concentration of from 12.5-19 grams/ litre of CoCl 6H O.

The other constituents of the plating bath can be suitably adjusted according to the ratio. of nickel salt to cobalt salt. The corresponding compositions of the coatings plated from these solutions are 65-83% nickel, 15- 33% cobalt and 2% phosphorus.

In the preferred example described above a copper sheet is used as the substrate and this is sensitised, i.e. catalytically activated, by touching it with an aluminum wire. Other substrates, metallic or non-metallic, may be used and appropriately activated by methods well known to those skilled in the art.

It is to be understood that the foregoing description of specific examples of this invention is made by way of 3 4 example only and is not to be considered as a limitation R f r Cited i m is. UNITED sTATEs PATENTS 1. A magnetic alloy consisting of about 65-83% nickel, 2,643,221 6/1953 Brenner 6t 31 75170 15-63% cobalt and 2% phosphorus, and which has a 5 2,644,737 7/1953 Bonn et a1 204-43 square hysteresis loop, a coercive force of not more than 3 3 9 2/1968 Schmeckenbechef 75170 oersteds, and is substantiall magneticall isotropic.

2. A magnetic alloy as cl aimed in clair ii 1, consisting RICHARD DEAN Pnmary Exammer of about 30% cobalt, 68% nickel and 2% phosphorus and U S C1 X R which has a coercive force of about 1 oersted and a 10 coefiicient of rectangularity of 0.95. 148-3155 

